The invention concerns a method and device for the measurement of currents in at least two measuring ranges in a conductor.
Optical measurement devices are known for measuring an electric current in a conductor using the Faraday effect, also referred to as magneto-optical current converters. The Faraday effect is the rotation of the polarization plane of linearly polarized light caused by a magnetic field. The rotation angle is proportional to the line integral over the magnetic field along the path traveled by light with the Verdet constant as the proportionality constant. The Verdet constant in general depends on the material, temperature, and wavelength. A Faraday element made of an optically transparent material, such as glass, for example, is arranged near the conductor to measure the current. The magnetic field generated by the current causes the plane of polarization of the linearly polarized light emitted by the Faraday element to rotate by an angle that can be analyzed as a measuring signal. In general, the Faraday element surrounds the conductor, so that the light used for measurement travels around the conductor in a basically closed path. The absolute value of the rotation angle in this case is directly proportional, with a good approximation, to the amplitude of the current to be measured. The Faraday element can be designed as a solid glass ring around the conductor, through which the light passes once only, or it can also surround the conductor in the form of a measuring coil made of a light-conducting monomode fiber (fiber coil). In a solid Faraday element, the measuring range of the current converter is adjusted by choosing the appropriate material; smaller Verdet constants are used for larger currents than those used for smaller currents. In the case of a measuring coil, the measuring range of the current converter can also be adjusted via the number of spires in the coil, since the Faraday rotation angle is also proportional to the number of spires, i.e., the number of turns of the light around the conductor. A current range over which the Faraday angle is a unique function of the current is selected as the measuring range. Since techically no distinction can be made between two polarization states of the measuring light that are antiparallel to one another, i.e., rotated by an angle .PI. in relation to one another, the measuring range of the magneto-optical current converter corresponds to a rotation angle interval with a maximum length of .PI./2.
In a magneto-optical current converter known from European Patent B-0,088,419, two Faraday glass rings are arranged in parallel around a common conductor. Both glass rings are made of Faraday materials with different measurement sensitivity and therefore different current measuring ranges. Each Faraday glass ring has a transmitter unit for transmitting linearly polarized measuring light into the glass ring, a Rochon prism, a Wollaston prism or another polarizing beam splitter as an analyzer to split the measuring light rotated after passing through the respective glass ring into two sub-beams with different polarization planes, photodiodes for converting the sub-beam signals of each of the two Faraday glass rings into electrical signals, as well as an analyzer unit to calculate the measurement signal for the corresponding Faraday rotation angle. The two measurement signals of the two Faraday glass rings are supplied to an OR gate, which provides a maximum signal of the two measurement signals. A larger measuring range can be covered with this maximum signal. Different measuring ranges of the two glass rings can also be achieved even using the same glass material for both glass rings, by using measuring light of different wavelengths. The wavelength-dependence of the Faraday rotation is made use of in this case.
For today's measuring technology, current converters for measuring and counting applications should be able to measure nominal currents in a predefined measuring range of up to 2000 A, for example, with a high measuring accuracy of typically between approximately 0.1% and approximately 0.5% and, for protection purposes, overcurrents in a measuring range of 10x to 30x, for example, the nominal current with a lower measuring accuracy of between approximately 5% and approximately 10%.
From "International Conference of Large High-Voltage Electric Systems," CIGRE, Paris, Aug. 28-Sep. 3, 1988, Conference Proceedings, T. Pref. Subj. 1, Vol. 34, Book 15, pp. 1-10, an optical fiber arrangement is known with a first magneto-optical current converter for measuring nominal currents in a measuring range of between 0.1x and 1x a predefined maximum nominal current with a measuring accuracy of 0.2% and with a second magneto-optical current converter for measuring overcurrents in a measuring range of between 0.1x and 20x the maximum nominal current with a measuring accuracy of 5%. The first current converter for measuring nominal currents is of the reflection type and comprises an optical fiber surrounding the conductor in the form of a measuring coil with N spires. Linearly polarized light from a light source is injected into the fiber through a beam splitter, traverses the measuring coil, is reflected back from a mirror in the fiber and traverses the coil for the second time in the reverse direction. Due to the non-reciprocity of the Faraday effect, the rotation angle is doubled. Due to the reciprocity of the intrinsic circular birefringence of the fiber material, the undesirable, in particular temperature-dependent, effects of the circular birefringence are eliminated. After traversing the measuring coil twice, the light is supplied to a Wollaston prism through the beam splitter, and split into two sub-beams polarized perpendicularly to one another, each of which is supplied to a light detector. A normalized intensity signal, equal to the quotient of the difference and the sum of the two intensity signals, is formed by an analyzer circuit from the two electrical intensity signals, formed by the light detector, corresponding to the sub-beam signals. The second magneto-optical current converter, provided for protection purposes, also comprises a light source, a polarizer, a fiber surrounding the conductor in the form of a measuring coil, and an analyzer circuit. Contrary to the first current converter provided for measuring purposes, the second urrent converter is of the transmission type, i.e., the light from the light source, linearly polarized in the polarizer, is injected into the measuring coil at one end, traverses the measuring coil once only, is removed from the other end of the measuring coil and supplied to the corresponding analyzer circuit. Furthermore, the measuring coil of the second current converter has only one measuring spire for adjustment to the overcurrent measuring range. The optical fiber is twisted in one direction of rotation along one half of the measuring spire and in the opposite direction of rotation in the other half of the spire to suppress its intrinsic circular birefringence (double-twisted fiber). In this known arrangement, each of the two current converters requires its own transmitter unit consisting of a light source and a polarizer for transmitting linearly polarized light.